It is well known to plate metal onto another surface. Of particular interest, is the recent desire to form copper surfaces on semiconductor surfaces in lieu of aluminum wiring. Copper plating has been considered as the most viable method of doing so.
The metal is deposited onto the silicon wafer by an electrochemical deposition process where the silicon wafer acts as the cathode and the copper or other insoluble metal acts as the anode. To obtain uniform copper deposition, uniform, high velocity fluid flow of the electrolyte and uniform electrical field are necessary to promote better mass transport, electrical current distribution to reduce additives consumption and to prevent anode passivation.
While the design of the plating cell and the fluid flow are critical to obtain desired plating uniformity, the design of the anode is also critical to the plating uniformity and low consumption of additives and energy. It is generally desirable for the anode to have uniform, high fluid flow throughout, a large anode surface area and a uniform electrical potential. Moreover for soluble copper anodes, uniform dissolution of copper to minimize change in anode shape is desired.
In order to accomplish these requirements, soluble copper anodes have been made of copper beads or shot which have been enclosed within a porous compartment. Alternatively, a solid copper or insoluble copper plate or disk has been used. In these designs, numerous holes or slots may be formed in order to create increased surface area and flow channels that allow for fluid flow through the anode. Alternatively, fluid may flow around the anode for agitation.
The approach using copper shot(s) in a casing is less desirable due to relatively poor electrical contact between the copper particles and the related anode buss as they are dissolved over time.
The approach using the metal disk or plate requires mechanical machining or some other technique to create the flow openings (holes or slots). This leads to the scraping of a large amount of valuable metal, Further, as the metal dissolves, the flow characteristics change as the holes or slots vary in width (typically going larger as the metal dissolves). Additionally, there is a delicate balance between the number of holes or slots formed in the metal disk or plate and the flow characteristic and plating uniformity obtained. If there are too few, one does not obtain the desired flow characteristics and plating uniformity. If there are too many or if the holes or slots are too big, the electrical field distribution is changed in an adverse way. Furthermore, since it is desirable to have similar geometry between the anode and the cathode for better electrical field distribution, the disk anode may be passivated at high speed plating applications. Typically the anode to cathode surface area ratio should be 2 to 3.
Lastly, in all of these approaches, there is no easy method to monitor the consumption of the anode over time.
What is desired is an anode that provides the uniform, high fluid flow, large surface area, minimum change in flow and electrical characteristics and uniform electrical field distribution with a means to monitor consumption over time in a plating system. The present invention provides such a device.
The present invention is a metal anode that has at least a portion formed in a spiral configuration with defined spacing between the adjacent spirals in order to provide fluid flow characteristics. Preferably, the anode is formed of one or more metal strips that are formed into a spiral pattern including a single spiral, a double spiral, serpentine spiral and a zigzag spiral. The strips may be made of metal rods or sheets. The strips may be relatively flat or may contain various surface patterns such as corrugated surfaces, grooves, holes or other such devices to enhance fluid flow. Preferably, the strips are wider than their thickness and the strips are longer than their width. The spiral configuration is formed either by casting, cutting or by winding the metal strip into the desired spiral pattern. Desirably, rods or screws may be inserted radially through the layers of the spiral in order to provide uniform spacing and or mechanical rigidity to the anode. Using metal rods or screws not only provides the spacing and rigidity but also helps to reduce the electrical resistance along the strips. Additionally, when electrical contacts are made at two locations of the spiral, it allows one to measure the change in resistance in the anode over time and thus monitor the condition of the anode so one may change the anode at the appropriate time. Lastly, one or more buss bars or electrical connections may be made to minimize voltage drop in the anode during use.
It is an object of the present invention to provide a metal anode comprising one or more metal strips at least a portion of which are formed into a spiral configuration and wherein each layer of the spiral is uniformity spaced apart from the adjacent layer of the spiral.
It is a further object of the present invention to provide a soluble anode comprising one or more metal strips, at least a portion of which are formed into a spiral configuration and wherein the anode contains a separate metal strip of the same metal as the anode which strip is used to monitor the consumption of the anode by electrical resistance measurement.
It is another object of the present invention to provide a system for electroplating comprising two or more cathodes formed of a material on which a metal is to be plated, two or more anodes formed of a metal from which the two or more cathodes are to be plated, said two or more anodes each having at least a portion being formed in a spiral configuration, wherein each layer of the spiral of each of the two or more anodes is uniformity spaced apart from the adjacent layers of the spiral, said two or more anodes being arranged such that the spiral configurations are parallel to the surface of the two or more cathodes and an electrolyte which flows through the spirals of the two or more anodes from a surface of the anode farthest from the two or more cathodes to the surfaces of the two or more cathodes.